Cooking apparatus of the type disclosed in U.S. Pat. No. 3,884,213 directed jets of temperature controlled gaseous fluids to irregular shaped food products to subject discrete area of the surface of the food product to a very high heat transfer rate. The areas subjected to the high heat transfer rate were moved over the surface of the product to obtain desired overall surface effects such as crisping, browning, searing or freezing, depending upon the temperature of the jets of gaseous fluid and the rate of movement of the jets across the surface of the food product.
U.S. Pat. No. 4,474,498 discloses a generally vertically disposed plenum extending along the back wall of an oven compartment, the compartment being divided into a plurality of horizontally spaced chambers and having a pair of conveyors moving therethrough, one above the other such that a food product was moved through the heating compartment by a conveyor making more than one pass through the compartment for controlling heat transferred to the product.
U.S. Pat. Nos. 4,154,861, 4,479,776 and 4,492,839 disclose a variety of fan, plenum and air dispensing devices to precisely control heat transfer to upper and lower surfaces of food products. The food products have been carried on a conveyor through tunnel ovens having air dispensing devices above and below the conveyor for top and bottom heat transfer to the food product.
The apparatus disclosed in the aforementioned patents generally incorporated a blower mounted either above or below a conveyor such that air impinging against the surface of the food product on the opposite side of the conveyor from the fan circulated partially through the conveyor and between air dispensing fingers to return to the suction side of the fan. However, when high air flows are applied or when the conveyor is completely covered by rectangular shaped pans or consists of a solid conveying band, the recirculating air had to pass beside the conveyor to return to the intake of the fan.
U.S. Pat. No. 4,523,391 disclosed a plenum having a plurality of inwardly inclined front wall panels adapted to direct the flow of air through an elongated plenum outlet port to air dispensing ducts positioned above and below a conveyor. Return ducts and the plenum were formed in a side of the housing adjacent one side of the conveyor permitting the flow of air generally transversely of the conveyor to return to the suction side of the fan.
The rate of air flow through ovens of the type hereinbefore described and disclosed in the aforementioned patents were somewhat limited by design considerations required to circulate air through the compartment without interfering with air streams employed for transferring heat to the surface of the food product carried by the conveyor. In certain applications difficulty was encountered in precisely controlling the rate of heat transfer to upper and lower surfaces of the food products since the devices relied primarily upon the volume of air delivered through ducts above and below the conveyor for transferring heat to upper and lower surfaces of the food product. The number of ducts was increased or decreased for adjusting the ratio of heat transferred to upper and lower surfaces of the food product.